Another important feature are the "super units". You should only use these upgrades if you have reached the population limit and if you are using at least 6 units of that kind. Then, these upgrades can make a huge difference, though.

Barracks

Upgrade to
Long Swordsman
80 HP

Upgrade toLegion
160 HP
requires Fanaticism

160
50

1400
600

Description: Like the Broad Sword upgrade, the Long Sword upgrade does not improve the strength of the Swordsmen very much. If you have chosen the Persian or Choson tribe, this upgrade is a must. Otherwise I would stick to Academy units since they are a lot stronger.
The Legion upgrade makes your men more than twice as strong; this upgrade is worth the price if you already have Long Swordsmen. If gold is scarce, trying your luck with Legions or Long Swordsmen instead of Centurions or Phalanxes may be better.

Storage pit

Metallurgy
+3 melee attack
Cataphract

Chain Mail
+2 Infantry armor

Chain Mail
+2 Archer armor
Heavy Horse Archer

Chain Mail
+2 Cavalry armor

Iron Shield
+1 piercing armor

300
180

125
100

150
100

175
100

200
320

Description: Get the Metallurgy upgrade because it helps your Infantry and your Cavalry alike. You get as much power as for the Metalworking upgrade.
Research only the Armors for the units of your choice. E. g. if your tribe has no Heavy Horse Archers, forget about that final armor upgrade.
The Iron Shield is expensive, but helps your infantry a lot against Archers and other missile weapons (e. g. towers and ships). Get it!

Each type of armor corresponds to the
building from which the units are trained. The Horse Archer, for example,
is both mounted and and an archer unit. Because it is trained at the
Archery Range, it utilizes Archer armor.

Mr. Grinch

Granary

Fortification
400 HP

Guard Tower
200 HP

Ballista Tower
200 HP
requires Ballistics

300
175

300
100

1800
750

Description: The upgrade to the Fortification is not cheap, but worth the price if you are towering. Get it if you want to build a Wonder.
The upgrade to the Guard Tower is relatively cheap and a must since it is the only way to make your towers effective against Triremes.
The upgrade to the Ballista Tower doubles the damage, but it is only worth the high cost if you play defensively (e. g. if you want to keep all artifacts or a Wonder).
The benefit is higher against units with a high piercing armor.

Archery

Heavy Horse Archer
90 HP, range: 7
requires Chain Mail for Achers

1750
800

Description: Horse Archers are already pretty good units and given for free to the 8 tribes that can get it (once you are at the Iron Age). They are better than Elephants Archers due to their higher damage and speed, making them excellent for hit and run attacks. If you keep your distance to Infantry units, they are nearly invulnerable.
The upgrade to the Heavy Horse Archer increases the HPs by 50% and the damage by 1, but leaves all other attributes unchanged. Since you will avoid direct combat anyway, the increased hit points are not very helpful. You should be happy with your normal Horse Archers and invest your gold elsewhere.

Stables

Heavy Cavalry
150 HP

Cataphract
180 HP
requires Metallurgy

350
125

2000
850

Description: The upgrade to the Heavy Cavalry is fairly expensive, but involves both standard and piercing armor upgrades (+1 each). Use your speed advantage for killing villagers quickly. You should get that upgrade if you have many Cavalry units. Cavalry is the peasant's bane :)
The upgrade to the Cataphract units involves few benefits (+2 damage, +2 standard armor) for a high cost. Make sure you have researched all Cavalry Armors before you get this upgrade, if you get it at all. Relying heavily on Cavalry units is only possible if you have a lot of gold.

Description: Again, Craftsmanship is the best upgrade here. Get it soon!
Siegecraft lets your villagers deal pretty much damage against towers and walls (about as much as Phalanxes do), but since that is still pretty much like suicide, you should forget about that and let the catapults do the job. The increased stone mining is useful for defensive strategies; get that upgrade if you need more stone. If you have a lot of surplus food and wood, you can get that upgrade for the sake of the Heavy Catapult.Coinage is a very good upgrade. It speeds up the gold mining, and the mines will actually yield 25% more gold than before. It also lets you trade resources with no additional costs (the tribute feature does not only have to be used for demanding tribute). Get it as soon as you can since the profit will diminish as you mine more gold!
Irrigation is pretty expensive and not worth it unless you need a lot of wood for ships. Just do not forget to switch back to the farms now and again and rebuild them when they are gone.

Dock

Heavy Transport
200 HP, +5 capacity

Trireme
200 HP

Catapult Trireme
120 HP
requires Trireme

Juggernaught
200 HP
+1 damage radius
requires Engineering

150
125

250
100

300
100

2000
900

Description: All these upgrades are well worth their price. You should get them all if possible.
The upgrade to the Heavy Transport makes your transport faster and lets it carry 10 units instead of 5.
A Trireme is almost twice as strong as a War Galley; this upgrade should be your first one on water maps!
A Catapult Trireme is the only way to fight against towers with ships. It has less hit points than a Trireme and thus should be escorted by Triremes.
As soon as Juggernaughts are invented, they will rule the sea. The only reason to build Triremes then is to spare gold. The upgrade requires a lot of food, but you have to get it on sea maps! This is, if you can get it; you should choose a tribe that allows Juggernaughts on sea maps. Like Heavy Catapults, Juggernaughts can destroy trees with "attack ground".

Temple

Afterlife
+3 Priest range

Monotheism
Priest can convert enemy
Priests and buildings

Fanaticism
Priest rejuvenation +40%
Legion

Jihad
see below

275

350

150

120

Description: An increased range is crucial when trying to convert slow units because it gives you more time. It also lets you convert all War ships from a safe distance.
Monotheism is very strong! It lets you take over enemy bases instead of destroying them. You still have to knock down towers because converting buildings does not work at a distance. Get this upgrade if you still have more than one town to fight against. Use a few Priests to take control of all buildings when the troops are defeated. Watch out for assassins, though!
Fanaticism is relatively cheap and lets you convert more than one unit in a battle. Your Priest gets his faith back in 30 instead of 50 seconds. Get this one!
Jihad ("Holy War") gives your villagers 65 HP and makes them very fast. However, their gathering abilities are reduced by up to 80%. Since your production will be crippled, the only strategy where Jihad makes sense is a Wonder strategy where you do not have enough troops to protect your town. Research Jihad when your Wonder is completed. You will get some cheap catapults killers then :)

Siege Workshop

Catapult
75 HP
damage radius: 2

Heavy Catapult
150 HP
damage radius: 3
requires Siegecraft

Helepolis
55 HP
fire rate x2
requires Craftsmanship

300
250

1800
900

1500
1000

Description: The Catapult increases the damage, range and damage area of your Siege Weapon. However, if your units stand near the target, they will now be hit as well due to the increased damage radius. You should get this upgrade, but watch out when your Catapults are firing!
The Heavy Catapult has more hit points and a larger damage radius.
This does usually not make a difference against buildings, but it lets you kill a lot of enemy units with one shot. You can also take down trees with Heavy Catapults (use "attack ground"), which might sometimes be useful.
However, you have to get Siegecraft to get this upgrade, and Siegecraft involves no benefits for an offensive strategy. This makes this upgrade too expensive unless you have surplus resources.

With double the hit points a Priest (for healing) and
Heavy Catapult combination can be all but unstoppable, unless that is, the
enemy deployes archers.

Mr. Grinch

Helepolis ("City killers") are just awesome against units. Make sure you have researched Ballistics before you get this upgrade. Due to its very high fire rate, a Helepolis can kill a lot of enemies before they can get through. Protect it with a few Phalanx units, and you will have an awesome attack force. If you build Ballistas, you should always keep an eye out on this upgrade.

Description: The Phalanx upgrade is not cheap, but still worth the cost if you have enough Hoplites. You should build two Academies to get Hoplites fast.
A Centurion is by far the strongest Infantry unit, it is more than twice as strong as a Phalanx. Get this upgrade if your tribe and your funds allow it, especially on land maps!

Government Center

Aristocracy
+25% speed of
Academy units
Centurion

175
150

Ballistics
Predictive firing
(see below)
Ballista Tower

200
50

Alchemy
+1 attack for
Missile units
and Siege Weapons

250
200

Engineering
+2 range for
Siege Weapons
Juggernaught

200
100

Description: The Iron Age is when the Government Center gets important since all upgrades that you can get here then are useful.
Aristocracy makes the slow but powerful Phalanxes faster and lets you upgrade to Centurion. Get it if you can afford the Centurion upgrade.
Ballistics is the most important one of these upgrades! With ballistics, your Siege Weapons will fire at the point where the enemy will be when the shot lands instead of where he is standing. This makes it a lot easier to hit moving targets.
Alchemy is only important if you have many Archers. +1 damage makes no difference for Siege Weapons.
Engineering is also a must! It will affect your Catapults, Ballista and Catapult Triremes (plus their upgraded versions). An increased range is very helpful. And it even lets you get Juggernaughts, by far the strongest ships at sea (like in Warcraft).

Special thanks go to Mr. Grinch for reviewing this page scrupulously and providing useful suggestions.